title: [Measuring intraocular pressure in keratoconus. Effect of the changed biomechanics].
abstract: BACKGROUND
  Keratoconus is associated with changes in the corneal structure, such as defects of Bowman's layer, a decrease of corneal thickness etc. They result in alterations of some of the biomechanical parameters of the cornea, namely, rigidity and elasticity. The present study was performed to examine how impression tonometry and applanation tonometry for determination of intraocular pressure (IOP) are affected by the changed biomechanical parameters associated with keratoconus.
  
  
  METHODS
  We examined 20 normal subjects (40 eyes) and 17 keratoconus patients (25 eyes). The corneal thickness was measured by ultrasound pachymetry in the corneal center. In the keratoconus patients, an additional measurement was made at the conus peak. The corneal curvature was determined using the TMS keratoscope. The IOP measurements were made with the Schiötz tonometer (10 g). For comparison, additional IOP measurements in the corneal center and, in the keratoconus group, on the conus peak were made with the applanation tonometer.
  
  
  RESULTS
  The normal subjects had a central corneal thickness of 548 +/- 30 microns, compared to 505 +/- 42 microns in the corneal center and 425 +/- 41 microns on the conus peak in keratoconus patients. The average corneal curvature was 43.3 +/- 1.8 D in the normal subjects and 47.8 +/- 4.1 D in keratoconus patients. Applanation tonometry produced results on 11.33 +/- 1.43 mm Hg in the normal group (corneal center) compared to values of 12.00 +/- 2.55 mm Hg (corneal center) and 7.30 +/- 1.95 mm Hg (conus peak) in the keratoconus cohort. The coefficient of rigidity was 0.0236 +/- 0.0026 microliter-1 in the normal subjects, compared to 0.0173 +/- 0.0050 microliter-1 in the keratoconus patients.
  
  
  CONCLUSIONS
  The morphological changes associated with keratoconus may cause tonometry errors.
authors.affiliation: Universitäts-Augenklinik Hamburg Eppendorf.
authors.forename: A
authors.initials: A
authors.lastname: Böhm
authors.forename: M
authors.initials: M
authors.lastname: Kohlhaas
authors.forename: R C
authors.initials: RC
authors.lastname: Lerche
authors.forename: B
authors.initials: B
authors.lastname: Bischoff
authors.forename: G
authors.initials: G
authors.lastname: Richard
mesh_terms: D000328:Adult; D015331:Cohort Studies; D003315:Cornea / Q000503:physiopathology; D004548:Elasticity; D005260:Female; D006801:Humans; D007429:Intraocular Pressure / Q000502:physiology*; D007640:Keratoconus / Q000503:physiopathology*; D008297:Male; D012016:Reference Values; D012029:Refraction, Ocular; D014065:Tonometry, Ocular
vernacular_title: Messung des intraokularen Druckes bei Keratokonus. Einfluss der veränderten Biomechanik.